Windchill Risk and Reliability Desktop Help > Windchill FRACAS > FRACAS Calculations > FRACAS Calculation Equations > MTBF (Mean Time Between Failures)
MTBF (Mean Time Between Failures)
MTBF is a basic measure of reliability for repairable items. The average time used in the MTBF is the time for which all of the parts of a defined item perform within their specified limits, during a particular measurement period, under stated conditions.
The equation for calculating MTBF is:
For system tree items that have zero failures, you can either ignore these items (default) or use of the Chi-Square distribution for zero failure calculations.
• If you ignore system tree items that have zero failures, the MTBF for these items is stored as a null value (not as zero).
• If you use of the Chi-Square distribution, you must define a confidence level between 0 and 100. The default confidence level is 75. The lower bound chi-square value is computed using two degrees of freedom. The equation for calculating MTBF for zero failures is:
For general theory associated with zero failure calculations, pages 63 through 72 in the following book are helpful: Applied Reliability, Second Edition, Paul A. Tobias and David C. Trindade.
The following table demonstrates the calculation and roll up of MTBF values when items that have zero failures are ignored.
System Tree Structure
Incidents
Time
Item MTBF
Rolled-UP MTBF
MTBF/60
System
0
1000
125
2.083333333
A1
0
1000
125
2.083333333
A1-S1
5
1000
200
200
3.333333333
A1-S2
3
1000
333.3333333
333.3333333
5.555555556
A2
0
1000
A2-S1
0
1000
This next table demonstrates the calculation and roll up of MTBF values when items having zero failures use the Chi-Square distribution. For more information, see Chi-Square Distribution.
System Tree Structure
Incidents
Time
Item MTBF
Rolled-UP MTBF
MTBF/60
System
0
1000
721.35
73.82711753
1.230451959
A1
0
1000
721.35
106.5383707
1.775639511
A1-S1
5
1000
200
200
3.333333333
A1-S2
3
1000
333.3333333
333.3333333
5.555555556
A2
0
1000
721.35
360.675
6.01125
A2-S1
0
1000
721.35
721.35
12.0225
Various MTBF values can also be calculated for a set of items by applying a filter to the System Tree Items table.